This invention relates to systems and methods for signal noise reduction in electrical systems. More particularly, the invention relates to systems and methods for signals such as those used in CATV upstream channels.
The demand for CATV bandwidth and types of signals transmitted on CATV is increasing. A typical cable TV system includes 80 or more analog TV channels, expandable in capacity to include 132 or more digital channels, data channels, and telephony signals. Analog video, QAM (quadrature amplitude modulation), QPSK (quadrature phase shift keying), digital video, data, and telephony signals are modulated on analog RF carriers. Digital video systems also provide video on-demand services.
One of the issues with modulated signals is signal noise. Signal noise effectively reduces the ability to transmit signals at low power levels and can cause distortion and error in demodulated output signals.
Upstream signals in a CATV system include a variety of signals, such as transmission of internet signals.
In CATV systems, there are multiple levels of carriers. In optically transmitted CATV signals, the optical transmission is a modulated carrier. The modulation is typically an RF signal which is a CATV carrier. The RF CATV carrier includes multiple channels, which themselves are carriers. When carriers are discussed herein, it is meant to include a signal which may be filtered in accordance with the invention. Examples of such carriers are EIA channels included in a CATV transmission and internet transmissions.
According to the present invention, a CATV filter circuit includes a detector circuit, an electronically adjustable adaptive high pass filter, and a control circuit. The detector circuit detects a lowest frequency used by signals in an upstream path, and the control circuit responds to the circuit for detecting. The adaptive high pass filter is electronically adjustable as a cutoff frequency. The control circuit responds to the circuit for detecting, and adjusts the cutoff frequency of the adaptive high pass filter to a frequency below the lowest frequency used by the signals in the upstream path. This has the affect of blocking ingress noise to an extent permitted by a frequency plan used in the upstream spectrum.
According to a particular aspect of the present invention, the CATV filter circuit uses artificial intelligence to further separate noise from signal by determining which transmissions are desired signals. Signals which do not meet predetermined criteria for signal transmission are then filtered. According to one embodiment of the present invention, the present criteria includes detection of a predetermined signal characteristic and a lowest frequency associated with the detection of a signal associated with that signal characteristic.
According to a further aspect of the present invention, the CATV filter circuit monitors signals in the upstream path on an ongoing basis. A control circuit uses two time constants, a first time constant for lowering the cutoff frequency and a second time constant for raising the cutoff frequency. This permits raising of the cutoff frequency after a time period consistent with termination of upstream path transmissions and lowering the cutoff frequency after a time period consistent with transmissions. The time period for lowering the cutoff frequency is sufficiently long to reduce false or spurious signal detection.
According to a further aspect of the present invention, the CATV filter curcuit includes a temperature compensation circuit.
According to a further aspect of the present invention, the CATV filter circuit includes a microprocessor. The microprocessor permits selection of signal criteria by determining which transmissions are desired signals and filtering transmissions which do not meet predetermined criteria for signal transmissions. The criteria includes at least detection of a signal characteristic and a lowest frequency associated with the detection of a signal associated with the predetermined signal characteristic.
According to a further aspect of the present invention, a high pass filter circuit and method are employed according to the present invention to reduce ingress transmission noise in a laser or other electronic system or device, and to reduce downstream distortion. According to one embodiment, the filtered signals are used to modulate a laser diode or an electrical system. The distortion signals compensate for anticipated distortions caused by the laser diode or other electrical system, leaving good quality signals such as CATV pictures, for example. According to a further aspect of the invention, a CATV transmitter is provided with a high pass filter.
One source of noise on CATV systems is low frequency noise. This originates from obvious sources such as 50/60 Hz AC transmission hum and from less obvious sources such as AM broadcast radio and other radio transmissions. The effect of such noise is a degradation of signal at desired modulation frequencies such as those used with QAM, QPSK, FM or PM signals.
While it is possible to establish fixed low frequency cutoffs, there are certain frequencies which are used in CATV transmissions and that are also particularly susceptible to low frequency noise.